1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image.
2. Description of the Related Art
There exists a tandem-type full-color image forming apparatus including a plurality of image forming units corresponding to development colors such as cyan, magenta, yellow, and black arranged thereon along a direction of movement of an intermediate transfer body.
As the tandem-type full-color image forming apparatus, there is known an apparatus which drives the image forming unit for black and the intermediate transfer body commonly by the same drive motor and the image forming units for the other colors by another drive motor. By driving the image forming unit for black by the drive motor different from the drive motor used to drive the image forming units for the other colors, the image forming units of the other colors can be held in a stopped state when the image forming apparatus is operated in a black monochromatic mode in which a monochromatic image is formed by using only the image forming unit for black.
In the full-color image forming apparatus, a color deviation between the colors is required to be reduced so as to enhance image quality. Therefore, rotation phases of photosensitive drums for the respective colors are required to be registered with each other. In particular, the photosensitive drum corresponding to black is driven by the drive motor different from that for the photosensitive drums corresponding to yellow, magenta, and cyan. Therefore, the rotation phase of the photosensitive drum corresponding to black and the rotation phases of the other photosensitive drums are required to be registered with each other.
An adjustment to register the rotation phases of the plurality of photosensitive drums takes long time. Therefore, if the rotation phases are adjusted at the start of image formation, it takes long time to start forming a print. Japanese Patent Application Laid-open No. 2006-58364 discloses a technology of detecting the rotation phases of the plurality of photosensitive drums to adjust the phases.
Toners (recording agents) used to form the images are collected from the photosensitive drums by cleaning blades so as not to remain thereon. However, when the image formation is not performed for a long period of time and therefore the photosensitive drums are not rotationally driven, a residual toner (residual recording agent) between the photosensitive drum and the cleaning blade sometimes firmly adheres to a surface of the photosensitive drum. In order to prevent the adhesion of the residual toner, in the image forming apparatus, microscale driving is performed for all the photosensitive drums at constant time intervals in the case where the image formation is not performed for a predetermined period of time.
Even when the photosensitive drums are stopped after the rotation phases of the photosensitive drums are registered with each other at the end of a printing operation as described in Japanese Patent Application Laid-open No. 2006-58364, there is a risk in that the rotation phases of the photosensitive drams are misregistered at the start of a next operation due to the microscale driving repeated at constant time intervals. If the photosensitive drums are activated in this state, the image quality is degraded because of a misregistration between the rotation phases of the photosensitive drums. Moreover, in the case where the rotation phases of the photosensitive drums are registered with each other again when the photosensitive drums are activated, it takes long time to start outputting the print.